In recent times, and with the application of new technologies, large passenger carrying marine craft are achieving speeds of a magnitude normally experienced in motor vehicles and so are capable of subjecting passengers seated within them to high lateral forces which can cause serious injury if the motion of the vessel is suddenly impeded.
One of the problems with designing leg attachment systems for seating in marine vessels, and which does not present itself to the same degree in relation to land based vehicles, is the weight consideration which seriously affects the design of the particular leg attachment system that is adopted. Moreover, in order to achieve high speeds, passenger ferry craft are required to be light weight, whereby the weight of the vessel directly affects the maximum speed that can be obtained by the vessel. Consequently, passenger ferry craft these days are constructed with decks of extremely light gauge material, for example aluminium sheeting which can reach thicknesses of down to 2 mm. Consequently, the leg attachment systems that have previously been designed for seating to attach to decks of such light gauge material have similarly been of light weight design and hence have skimped on meeting minimum safety standards for resisting laterally applied impact forces.
An immediate solution to the problem would be to make leg attachment systems of heavier gauge material, however the inherent problem with this is that this would considerably add to the weight of the vessel.
A further problem is that the beam of marine craft is not limited to the same extent as the breadth of land based vehicles and hence there is much more flexibility in the positioning and the number of seats that are able to be mounted upon a deck than is the case with land based motor vehicles. Hence, the ability to position seats easily and removably is also an important design consideration which detracts from having permanent fixed point anchoring systems for leg attachment systems, which are easier to design for meeting high loading requirements than is the case of impermanent anchoring points and fixtures.
Present types of anchoring techniques for leg attachment systems are basically divided into three classes. The first is the flange or base plate anchoring system as shown in FIG. 1A of the accompanying drawings, the second is the block mounted anchoring system as shown in FIG. 1B of the drawings, and the third is the track mounted anchoring system using either diamond or T-head bolts (not shown) or angle bolts shown in FIG. 1C.
As shown in FIG. 1C, this system uses an interlocking angle `a` to which tie bolts `b` are fixedly mounted to fixedly retain a seat pedestal leg `c` to a track `d`. The track `d` itself is permanently fixed to the deck and is formed with a longitudinally extending cavity, centrally disposed within and circumscribed by a C-shaped channel portion `e`. Such an arrangement has the advantage that the interlocking angle can be positioned close to the installation site without having to be slid along the entire length of deck track, which would otherwise be the case if a captive T-head section was used. Although a T-head section would provide greater locking ability and hence strength, as lengths of track can be as long as 50 m or more along a deck, such a system is impractical to use.
The first two of these systems falls into the category of permanent affixture anchoring points and is not attractive to the industry due to the inability to change the fixing position of the leg attachment for the seats, once it is fixed in position. A further problem is that almost all of the impact force is directly transferred to the deck of the vessel at the fixing point of the bolts, in the case of the flange or base plate arrangement, and the blocks, in the block mounted arrangement. Such forces consequently impose considerable shear upon the deck itself and in the case of lightweight thin gauge deck construction, the anchoring systems are able to pull out directly from the deck at relatively low impact forces, causing permanent damage to the deck itself.
The track arrangement, however, is generally accepted as the industry standard, whereby the track system allows for easy and impermanent positioning of the anchoring means therealong, whilst causing the impact forces to be applied to the track itself rather than the deck. Accordingly, the track system allows the leg attachments for seats to be easily slid into position, and the track itself can be attached to the deck using various fixing systems, including rivet and nut inserts which allow forces transmitted to the track to be more uniformly spread along the deck, reducing shear. In addition, the track provides a continuous attachment method for the leg attachments of the seats to the deck, where the leg attachments can be adjustably positioned at any time by sliding the legs in the track to the correct position.
Notwithstanding this improved arrangement, the two types of anchoring systems used with the track arrangement, namely the diamond or T-head bolt anchoring system or the angle bolt anchoring system, have been found to be limited with respect to the size of the impact force that can be accommodated and fall short of the standards recently set by the International Maritime Organisation which are applicable to high speed passenger carrying vessels.
Moreover, in the case of the diamond or T-head bolt arrangements, impact forces are still transmitted by the anchoring system to the track at point locations, consequently causing shearing of the anchoring means from the track at these points. In the case of the angle bolt attachment system, although impact forces are applied more uniformly along the angle, due to the limited ability of the angle to lock into the cavity of the C-shaped channel design of the track, the angle itself has a tendency to pull out or shear from the track by the impact force, creating a bending moment around the flange or lip `f` of the track which engages the flange of the angle.
Safety standards imposed by the International Maritime Organisation are calculated in accordance with a vessel's "G" rating. Moreover, each type of vessel has a "G" rating associated therewith which equates to the likely inertial forces that may be applied to passengers, having regard to the vessel size, its speed and weight.
"G" ratings for most large passenger carrying marine craft require leg attachment systems to resist impact forces in the region of 600 kg. With respect to the previously described prior art leg attachment systems, impact force resistance of only 300 kg to 400 kg can be achieved before experiencing vertical separation of the leg attachment from the deck or the track.